300
C. G. ST. C. Kendall· A. Sen
tive amounts of back reef and talus deposition, windward versus leeward effects
were modeled. The angle of repose of 20° was important for deposition of these
mass gravity flows. Carbonates have a high angle of repose, which decreases
with cohesiveness (Kenter and Schlager 1989).
Experiments with the Simulation. Several experiments were performed in order to
fine tune the simulation. Once the shape of the initial basin surface was determined, the elevation of that surface had to be positioned in such a way that at the
end of the simulation run there was an exact fit of the sedimentary fill of each of
the sequences to the accommodation. Using Haq et al. (1987) sea-level curve as input, this match was dependent upon both subsidence rates and rates of carbonate
accumulation. Attempts were made to keep the subsidence constant through several sea-level cycles. Ultimately a value of 0.009 m/1000 years was selected and this
value was kept constant from 30-10 Ma. Thereafter, the subsidence rate had to be
increased, especially after 6 Ma, to enable the exact fill of sediments for that time
interval. Additionally erosional surfaces were created at the interpreted type 1 sequence boundaries of 10.5 and 3.5 Ma. These erosional events are obvious on the
seismic and the 3.5 Ma boundary has a particularly irregular erosional surface.
Once subsidence was determined, the rate of benthic carbonate accumulation
and that of pelagic deposition were established individually and coordinated with
each other. Then two sets of experiments were performed. In one, the benthic
carbonate rates were varied with a constant rate of pelagic accumulation, and in
another, with slightly different constant values of benthic carbonate rates, the rate
of pelagic accumulation was changed. In each case the resulting simulation was
matched step by step with the seismic interpretation.
Though the rates of accumulation for pelagic and benthic carbonate rates were
varied considerably, the experiments showed that once a combination of the two
sources of carbonate produced the best match between the simulation and seismic, it was very difficult, if not impossible to further change the rates to produce
better matching simulation outputs. This combination of benthic and pelagic
rates produced the correct dimensions for the progradation and aggradation of
the sediment wedges without oversteepening the prograding clinoforms. The
timing and sequences of progradation was controlled by the size and timing of
the sea-level excursions. Attempts were also made to develop similar geometries,
with different sea-level curves and different values of same input parameters.
While it was possible to produce similar geometries it was, however, not possible
to match the output to the seismic interpretation.
3.4
Results from the Bahamas Simulation Execution
When the simulation is examined in detail, it is possible to see that each one of
the sequences produced, matches those on the seismic (Fig. 6). These are in order, sequences from 30 to 28.5 Ma 28.5 to 25.5 Ma; from 25.5 to 22.5 Ma; from
22.5 to 21 Ma; from 21 to 17.5 Ma; from 17.5 to 16.5 Ma; from 16.5 to 15.5 Ma;
C. G. ST. C. Kendall· A. Sen
tive amounts of back reef and talus deposition, windward versus leeward effects
were modeled. The angle of repose of 20° was important for deposition of these
mass gravity flows. Carbonates have a high angle of repose, which decreases
with cohesiveness (Kenter and Schlager 1989).
Experiments with the Simulation. Several experiments were performed in order to
fine tune the simulation. Once the shape of the initial basin surface was determined, the elevation of that surface had to be positioned in such a way that at the
end of the simulation run there was an exact fit of the sedimentary fill of each of
the sequences to the accommodation. Using Haq et al. (1987) sea-level curve as input, this match was dependent upon both subsidence rates and rates of carbonate
accumulation. Attempts were made to keep the subsidence constant through several sea-level cycles. Ultimately a value of 0.009 m/1000 years was selected and this
value was kept constant from 30-10 Ma. Thereafter, the subsidence rate had to be
increased, especially after 6 Ma, to enable the exact fill of sediments for that time
interval. Additionally erosional surfaces were created at the interpreted type 1 sequence boundaries of 10.5 and 3.5 Ma. These erosional events are obvious on the
seismic and the 3.5 Ma boundary has a particularly irregular erosional surface.
Once subsidence was determined, the rate of benthic carbonate accumulation
and that of pelagic deposition were established individually and coordinated with
each other. Then two sets of experiments were performed. In one, the benthic
carbonate rates were varied with a constant rate of pelagic accumulation, and in
another, with slightly different constant values of benthic carbonate rates, the rate
of pelagic accumulation was changed. In each case the resulting simulation was
matched step by step with the seismic interpretation.
Though the rates of accumulation for pelagic and benthic carbonate rates were
varied considerably, the experiments showed that once a combination of the two
sources of carbonate produced the best match between the simulation and seismic, it was very difficult, if not impossible to further change the rates to produce
better matching simulation outputs. This combination of benthic and pelagic
rates produced the correct dimensions for the progradation and aggradation of
the sediment wedges without oversteepening the prograding clinoforms. The
timing and sequences of progradation was controlled by the size and timing of
the sea-level excursions. Attempts were also made to develop similar geometries,
with different sea-level curves and different values of same input parameters.
While it was possible to produce similar geometries it was, however, not possible
to match the output to the seismic interpretation.
3.4
Results from the Bahamas Simulation Execution
When the simulation is examined in detail, it is possible to see that each one of
the sequences produced, matches those on the seismic (Fig. 6). These are in order, sequences from 30 to 28.5 Ma 28.5 to 25.5 Ma; from 25.5 to 22.5 Ma; from
22.5 to 21 Ma; from 21 to 17.5 Ma; from 17.5 to 16.5 Ma; from 16.5 to 15.5 Ma;